In recent years, a non-aqueous electrolyte secondary battery, which employs a non-aqueous electrolyte wherein lithium ion is moved between a positive electrode and a negative electrode to perform charging and discharging, has been widely used as a new type of secondary battery featuring high power and high energy density.
Such a non-aqueous electrolyte secondary battery generally employs a non-aqueous electrolyte comprising a mixed solvent wherein cyclic carbonic acid ester, such as ethylene carbonate, and chain carbonic acid ester, such as diethyl carbonate, ethyl methyl carbonate, and dimethyl carbonate, are mixed in a non-aqueous solvent, and an electrolyte of lithium salt, such as LiPF6 or LiBF4, dissolved in the mixed solvent.
However, the non-aqueous electrolyte secondary battery using the above-described non-aqueous electrolyte has a problem as follows. When a storage test where the non-aqueous electrolyte secondary battery is left in charging condition under high temperature is conducted for evaluating durability, a side reaction wherein the non-aqueous electrolyte reacts with the positive electrode and the negative electrode is caused, and battery capacity is decreased.
Therefore, it has disclosed that a variety of chain fluorinated carboxylic acid ester is used as a non-aqueous solvent of a non-aqueous electrolyte or as an additive to the non-aqueous electrolyte (See patent documents 1-5).
Generally, if the solvent is fluorinated, oxidation-resistance is improved, and as a result, a reaction between a positive electrode and a non-aqueous electrolyte is suppressed. However, if fluorine is introduced, viscosity of the non-aqueous electrolyte is increased, and rise of reactivity with a negative electrode is caused because of lowering of reduction-resistance. Especially, the reactivity with negative electrode is greatly influenced by a position where fluorine is introduced.
However, in each patent document, types of chain fluorinated carboxylic acid ester used are varied. As to the position where carbon is fluorinated, the patent documents 1 and 2 only suggest that hydrogen on α carbon is replaced with fluorine, the patent documents 3 and 4 suggest that any of α carbon and other carbon may be used, and the patent document 5 suggests it is preferable to replace hydrogen on α carbon with fluorine.
Further, if chain fluorinated carboxylic acid ester wherein hydrogen on α carbon is replaced with fluorine, for example, ethyl trifluoroacetate CF3COOCH2CH3, is used as the non-aqueous solvent of the non-aqueous electrolyte, lithium salt, such as LiPF6, as electrolyte, is not dissolved properly. If ethyl difluoroacetate CHF2COOCH2CH3 is used, although lithium salt, such as LiPF6, as an electrolyte is dissolved, the reactivity with the negative electrode is risen, and battery capacity is greatly decreased and battery characteristics are deteriorated in the case that the non-aqueous electrolyte secondary battery is left in charging condition under high temperature. As a result, the use of chain fluorinated carboxylic acid ester wherein hydrogen on α carbon is replaced with fluorine can not attain sufficient battery characteristics.
Still further, if chain fluorinated carboxylic acid ester wherein hydrogen on carbon which is not α carbon is replaced with fluorine is used as the non-aqueous solvent of the non-aqueous electrolyte, the reactivity with the negative electrode is lowered. However, in the case that the non-aqueous electrolyte secondary battery is left in charging condition under high temperature, there remain problems that the battery capacity is decreased and the battery characteristics are deteriorated. Also, when such a chain fluorinated carboxylic acid ester is used in combination of another non-aqueous solvent, if the non-aqueous solvent to be combined is not appropriate, initial capacity of the non-aqueous electrolyte secondary battery is decreased. Furthermore, in the case that the non-aqueous electrolyte secondary battery is left in charging condition under high temperature, the battery capacity and the battery characteristics are decreased.
As described above, although fluorination of non-aqueous electrolyte contributes to suppression of the reaction between the positive electrode and itself, the reactivity between the negative electrode and itself is increased, so that excellent battery characteristics can not be obtained.
[Patent Document 1] JP-A 8-298134
[Patent Document 2] JP-A 11-86901
[Patent Document 3] JP-A 6-20719
[Patent Document 4] JP-A 2003-282138
[Patent Document 5] JP-A 2006-32300